Electrical insulating oil composition

ABSTRACT

An electrical insulating oil having a flash point (PMCC) of 130° C. or more and a pour point of −40° C. or lower, comprising hydrorefined mineral oil and/or synthetic hydrocarbon oil and in the range of from 0.5 to 10 wt. % of bright stock oil, based on the total weight of the electrical insulating oil composition.

RELATED CASES

The present application claims priority from Japanese application 2006-039407, filed Feb. 16, 2006, which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an electrical insulating oil composition.

BACKGROUND OF THE INVENTION

Transformers, high-pressure circuit breakers and other high-pressure electrical devices are filled with electrical insulating oil compositions before use. However, it must be possible for such electrical devices to be used maintenance-free for long periods after the start of operation. Similarly, such compositions are required to maintain stable physical properties and electrical properties over a long period.

It is known that when electrical insulating oil compositions are used for a long period, they deteriorate, causing a decrease in anti-corrosion properties and a decrease in insulating properties due to an increase in the acid value of the electrical insulating oil compositions and the formation of sludge. Hence, with the trend towards higher pressure, larger size and longer service life for high pressure electrical devices, the stability of electrical insulating oil compositions used therein has become more important.

The IEC (International Electrotechnical Commission) has specified the required performance for electrical insulating oil compositions used for filling transformers, high pressure circuit breakers and other electrical devices, and the performance required for mineral oil insulating oil compositions capable of withstanding long-term use as the standards IEC 296 and IEC 60296. Meanwhile, with the advances in mineral oil refining technology, the hydrorefining method has been widely adopted, making it possible to obtain oils from which polycyclic aromatics, unsaturated components, nitrogen content and sulphur content have been adequately removed. At first, it had been considered that it was better for electrical insulating oil composition to contain no nitrogen or sulphur content.

Subsequently, however, Japanese Laid-Open Patent Application No. 2000-345177 A disclosed that with regard to oxidation stability and other aspects it was preferable for electrical insulating oil compositions to contain a certain amount of these components. Thus, it has been proposed in JP 2000-345177 A that by reducing the resin content in an electrical insulating oil composition to ≦100 ppm, and also incorporating minute amounts of sulphide-type sulphur content and total nitrogen content, the stability of the electrical insulating oil composition is increased, and increases in electric charge are inhibited.

In fact, it is extremely difficult to refine a crude oil by petroleum refining processes to a state wherein the nitrogen content or sulphur content are such as to be suitable for use in an electrical insulating oil composition. Accordingly, at present, although there have been attempts to adjust the components therein by adding an appropriate combination of additives or the like having a nitrogen content or sulphur content to highly purified refined oils, there are restrictions as to the additives which can be used, depending on the electrical insulating oil specification, moreover, there are cost problems, and production management complications.

It is therefore highly desirable to be able to obtain an electrical insulating oil composition having a balance of excellent electrical properties, low temperature performance, thermal oxidation stability and anti-corrosion properties by an economical and simple method.

SUMMARY OF THE INVENTION

In the present invention it has been surprisingly found that, through the addition of small quantities of bright stock oil to hydrorefined mineral oil and/or synthetic hydrocarbon oil, it is possible simply and economically to increase the thermal oxidation stability and obtain electrical insulating oil compositions which are capable of fulfilling the various types of performance required of electrical insulating oil compositions in the IEC Standard 296 and the IEC Standard 60296.

Accordingly, the present invention provides an electrical insulting oil composition having a flash point (PMCC) of 130° C. or more and a pour point of −40° C. or lower, which composition comprises hydrorefined mineral oil and/or synthetic hydrocarbon oil and in the range of from 0.5 to 10 wt. % of bright stock oil, based on the total weight of the electrical insulating oil composition.

In a preferred embodiment of the present invention, the electrical insulating oil composition has a pour point of −45° C. or lower.

In a preferred embodiment of the present invention, the electrical insulating oil composition has a flash point (PMCC) of 135° C. or more.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The hydrorefined mineral oil that may be present in the electrical insulating oil composition of the present invention may be refined by hydrogenating mineral oil. Examples of hydrorefined mineral oils that may be conveniently used include hydrorefined naphthenic mineral oil and hydrorefined paraffinic mineral oil.

Through the hydrorefining of mineral oils, polycyclic aromatic substances, unsaturated components, sulphur compounds and nitrogen compounds are preferably removed. Compared to the hydrorefined paraffinic mineral oils, the hydrorefined naphthenic mineral oils may have better low temperature flow properties and lower oxidation stability, and, conversely, compared to the hydrorefined naphthenic mineral oils, the hydrorefined paraffinic mineral oils may have inferior low temperature flow properties, but better thermal oxidation stability.

Examples of synthetic hydrocarbon oils that may be conveniently used in the electrical insulating oil composition of the present invention include polypropylene, polybutene, polypentene and other isoparaffinic synthetic hydrocarbon oils, poly-α-olefins (PAO) falling within Group IV in the API (American Petroleum Institute) base oil classification stated below in Table 1, and Gas to Liquid (GTL) synthetic oils. Synthetic hydrocarbon base oils sold by the Shell group under the trade designation “XHVI” may be conveniently used.

Japanese Laid-Open Patent Application 52-072706 A and Japanese Laid-Open Patent Application 07-228876 describe methods of manufacturing synthetic hydrocarbon oils that may be conveniently used in the electrical insulating oil composition of the present invention.

The afore-mentioned hydrorefined mineral oils and synthetic hydrocarbon oils may be used alone. However, the pour point, kinematic viscosity, thermal oxidation stability and the like of the electrical insulating oil composition of the present invention may be optimally adjusted by appropriate combination of several substances.

The hydrorefined mineral oil and/or synthetic hydrocarbon oil used in the electrical insulating oil composition of the present invention each have a preferred kinematic viscosity at 40° C. in the range of from 8.0 to 24 mm²/s.

Preferred hydrorefined mineral oils may be selected from hydrorefined naphthenic mineral oils and/or API base oil classification Group II or Group III hydrorefined paraffinic mineral oils.

Preferred synthetic hydrocarbon oils may be selected from isoparaffinic synthetic oils, GTL synthetic oils and poly-α-olefins (PAO) belonging to API base oil classification Group IV or mixtures thereof.

Since the kinematic viscosity of bright stock oil is relatively high compared to hydrorefined mineral oils and/or synthetic hydrocarbon oils, it is particularly preferred that the intrinsic kinematic viscosity of the hydrorefined mineral oil and/or synthetic hydrocarbon oil at 40° C. is 16 mm²/s or less, more preferably 12 mm²/s or less if hydrorefined mineral oil or synthetic hydrocarbon oil is used alone with bright stock oil.

If combinations of hydrorefined mineral oil and synthetic hydrocarbon oil are used with bright stock oil, the respective kinematic viscosity of the hydrorefined mineral oil and the synthetic hydrocarbon oil is each preferably in the range of from 8.0 to 24.0 mm²/s, more preferably in the range of from 8.0 to 20.0 mm²/s. TABLE 1 Saturated Sulphur content Viscosity Class content wt. % wt. % index Group I  >0.03  <90 80-120 Group II ≦0.03 ≧90 80-120 Group III ≦0.03 ≧90 ≧120 Group IV poly-α-olefin Group V Substances not belonging to Groups I-IV (esters, etc.)

The bright stock oil added to the afore-mentioned hydrorefined mineral oil and/or synthetic hydrocarbon oil may be obtained by further distilling under reduced pressure the atmospheric pressure residual mineral oil obtained when paraffinic mineral oil is distilled at atmospheric pressure from crude oil, and performing deasphalting, solvent extraction and solvent dewaxing processes on that reduced pressure residual oil. That is to say, said bright stock oil may be a heavy mineral oil base oil fraction obtained by reduced pressure distillation of the residual paraffinic mineral oil from atmospheric distillation of crude oil, and deasphalting, solvent extraction and solvent dewaxing treatment of the residual oil obtained.

The bright stock oil used in the present invention is preferably a base oil belonging to API base oil classification Group I, with total sulphur content in the range of from 0.4 to 2.0 wt. %, mercaptan sulphur content of 100 wt. ppm or less, sulphide-type sulphur content of 0.5 wt. % or less, total nitrogen content in the range of from 100 to 500 wt. ppm, basic nitrogen content of 300 wt. ppm or less, and total content of polar components (IP368) in the range of from 30 to 70 wt. %.

Various sulphur compounds and nitrogen compounds are concentrated in said bright stock oil by the distillation and solvent extraction processes. Through the use of in the range of from 0.5 to 10 wt. % of said bright stock oil in the electrical insulating oil composition of the present invention, preferably in the range of from 0.5 to 8.0 wt. %, based on the total weight of said electrical insulating oil composition, the afore-mentioned sulphur content and nitrogen content are incorporated in appropriate quantities in the electrical insulating oil composition. Hence, it is possible to markedly improve the thermal oxidation stability thereof. If the afore-mentioned mercaptan sulphur and sulphide-type sulphur contents become too high, then the corrosivity may become higher, which is undesirable. Further, if the basic nitrogen content exceeds 300 wt. ppm, then the oxidation stability performance may decrease.

The electrical insulating oil composition of the present invention has a preferred kinematic viscosity at 40° C. in the range of from 8.0 to 16.0 mm²/s, more preferably in the range of from 8.0 to 12.0 mm²/s. With a kinematic viscosity at 40° C. of less than 8.0 mm²/s, a satisfactory flash point may not be obtained. If the kinematic viscosity at 40° C. exceeds 16.0 mm²/s, then the circulation efficiency of the electrical insulating oil composition in a transformer may decrease, the cooling effect may become inadequate, and overheating may occur.

The electrical insulating oil composition of the present invention may comprise one or more antioxidant additives. Antioxidants that may be conveniently used are phenolic or aminic antioxidants. Examples of phenolic antioxidants that may be conveniently used in the electrical insulating oil composition of the present invention include 4,4′-methylenebis(2,6-di-tert.-butylphenol), 4,4′-bis(2,6-di-tert.-butylphenol), 4,4′-bis(2-methyl-6-tert.-butylphenol), 2,2′-methylenebis(4-ethyl-6-tert.-butylphenol), 2,2′-methylenebis(4-methyl-6-tert.-butylphenol), 4,4′-butylidenebis(3-methyl-6-tert.-butylphenol), 4,4′-isopropylidenebis(2,6-di-tert.-butyl-phenol), 2,2′-methylenebis(4-methyl-6-nonylphenol), 2,2′-isobutylidenebis(4,6-dimethyl-phenol), 2,2′-methylenebis(4-methyl-6-cyclohexylphenol), 2,6-di-tert.-butyl-4-methylphenol, 2,6-di-tert.-butyl-4-ethylphenol, 2,4-dimethyl-6-tert.-butylphenol, 2,6-di-tert.-butyl-α-dimethylamino-p-cresol, 2,6-di-tert.-butyl-4(N,N′-dimethylaminomethylphenol), 4,4′-thio-bis(2-methyl-6-tert.-butylphenol), 4,4′-thiobis(3-methyl-6-tert.-butylphenol), 2,2′-thiobis-(4-methyl-6-tert.-butylphenol), bis(3-methyl-4-hydroxy-5-tert.-butylbenzyl) sulphide, bis(3,5-di-tert.-butyl-4-hydroxybenzyl)sulphide, 2,2′-thio-diethylenebis[3-(3,5-di-tert.-butyl-4-hydroxyphenyl)propionate], tridecyl-3-(3,5-di-tert.-butyl-4-hydroxyphenyl)propionate, pentaerythrityl-tetrakis[3-(3,5-di-tert.-butyl-4-hydroxyphenyl)propionate], octyl-3-(3,5-di-tert.-butyl-4-hydroxyphenyl)propionate, octadecyl-3-(3,5-di-tert.-butyl-4-hydroxyphenyl)propionate, octyl-3-(3-methyl-5-tert.-butyl-4-hydroxyphenyl)propionate.

The content of the one or more antioxidant additives is preferably less than 2 wt. %, more preferably less than 1 wt. %, even more preferably less than 0.6 wt. % and most preferably less than 0.4 wt. %, based on the total weight of the electrical insulating oil composition.

In a preferred embodiment of the present invention, the electrical insulating oil composition comprises in the range of from 0.01 to 0.4 wt. % of phenolic antioxidant, based on the total weight of the electrical insulating oil composition. In other embodiments of the present invention, it is preferred that the electrical insulating oil composition comprises in the range of from 0.01 to 0.08 wt. % or in the range of from 0.08 to 0.4 wt. % of phenolic antioxidant, based on the total weight of the electrical insulating oil composition.

Further additives that may also be present in the electrical insulating oil composition of the present invention are dependent upon the specific application of the electrical insulating oil composition.

Through the addition of bright stock oil to the afore-mentioned hydrorefined mineral oil and/or synthetic hydrocarbon oil, it is possible to obtain electrical insulating oil compositions conforming with IEC (International Electrotechnical Commission) Standard 296, Class II, in that on maintaining for 168 hrs at 100° C. in IEC oxidation stability test method 61125A, sludge formation is 0.1 wt. % or less, and the acid value is 0.4 mg KOH/g or less.

Accordingly, in a preferred embodiment, the present invention provides an electrical insulating oil composition having a flash point (PMCC) of 130° C. or more and a pour point of −40° C. or lower, more preferably of −45° C. or lower, which composition comprises hydrorefined mineral oil and/or synthetic hydrocarbon oil and in the range of from 0.5 to 10 wt. % of bright stock oil, and which composition conforms with IEC (International Electrotechnical Commission) Standard 296, Class II, in that after 168 hrs at 100° C. in IEC oxidation stability test method 61125A, sludge formation is 0.1% or less, and the acid value is 0.4 mg KOH/g or less.

Further, through the addition of bright stock oil to the afore-mentioned hydrorefined mineral oil and/or synthetic hydrocarbon oil, it is possible to obtain electrical insulating oil compositions conforming with IEC Standard 60296, Type U, in that on maintaining for 164 hrs at 120° C. in IEC oxidation stability test method 61125C, sludge formation is 0.8 wt. % or less, and the acid value is 1.2 mg KOH/g or less.

Accordingly, in a preferred embodiment, the present invention further provides an electrical insulating oil composition having a flash point (PMCC) of 130° C. or more, more preferably of 135° C. or more and a pour point of −40° C. or lower, more preferably of −45° C. or lower, which composition comprises hydrorefined mineral oil and/or synthetic hydrocarbon oil and in the range of from 0.5 to 10 wt. % of bright stock oil, and which composition conforms with IEC Standard 60296, Type U, in that after 164 hrs at 120° C. in IEC oxidation stability test method 61125C, sludge formation is 0.8 wt. % or less, and the acid value is 1.2 mg KOH/g or less.

Moreover, in a preferred embodiment through the addition of bright stock oil, and the further addition of in the range of from 0.01 to 0.4 wt. %, more preferably in the range of from 0.08 to 0.4 wt. % of an antioxidant, preferably a phenolic antioxidant, to the afore-mentioned hydrorefined mineral oil and/or synthetic hydrocarbon oil, it is possible to obtain electrical insulating oil compositions conforming with IEC Standard 60296, Type I, in that on maintaining for 500 hrs at 120° C. in IEC oxidation stability test method 61125C, sludge formation is 0.8 wt. % or less, and the acid value is 1.2 mg KOH/g or less.

Accordingly, in a preferred embodiment, the present invention further provides an electrical insulating oil composition having a flash point (PMCC) of 130° C. or more, more preferably of 135° C. or more and a pour point of −40° C. or lower, more preferably of −45° C. or lower, which composition comprises hydrorefined mineral oil and/or synthetic hydrocarbon oil, in the range of from 0.5 to 10 wt. % of bright stock oil and in the range of from 0.01 to 0.4 wt. %, more preferably in the range of from 0.08 to 0.4 wt. % of an antioxidant, preferably a phenolic antioxidant and which composition conforms with IEC Standard 60296, Type I, in that after 500 hrs at 120° C. in IEC oxidation stability test method 61125C, sludge formation is 0.8 wt. % or less, and the acid value is 1.2 mg KOH/g or less.

Further, in the same way, through the addition of bright stock oil, and the further addition of in the range of from 0.01 to 0.4 wt. %, more preferably in the range of from 0.01 to 0.08 wt. % of an antioxidant, preferably a phenolic antioxidant, to the afore-mentioned hydrorefined mineral oil and/or synthetic hydrocarbon oil, it is possible to obtain electrical insulating oil compositions conforming with IEC Standard 60296, Type T, in that on maintaining for 332 hrs at 120° C. in IEC oxidation stability test method 61125C, sludge formation is 0.8 wt. % or less, and the acid value is 1.2 mg KOH/g or less.

Accordingly, in a preferred embodiment, the present invention further provides an electrical insulating oil composition having a flash point (PMCC) of 130° C. or more, more preferably of 135° C. or more and a pour point of −40° C. or lower, more preferably of −45° C. or lower, which composition comprises hydrorefined mineral oil and/or synthetic hydrocarbon oil, in the range of from 0.5 to 10 wt. % of bright stock oil and in the range of from 0.01 to 0.4 wt. %, more preferably in the range of from 0.01 to 0.08 wt. % of an antioxidant, preferably a phenolic antioxidant and which composition conforms with IEC Standard 60296, Type T, in that after 332 hrs at 120° C. in IEC oxidation stability test method 61125C, sludge formation is 0.8 wt. % or less, and the acid value is 1.2 mg KOH/g or less.

That is to say, the present invention provides an electrical insulating oil composition which conforms to one or more of the following standards:—IEC (International Electrotechnical Commission) Standard 296 (Class II), IEC Standard 60296 (Type U), IEC Standard 60296 (Type I) and IEC Standard 60296 (Type T).

The present invention further provides for the use of an electrical insulating oil composition as hereinbefore described in order to conform with one or more of IEC (International Electrotechnical Commission) Standard 296 (Class II), IEC Standard 60296 (Type U), IEC Standard 60296 (Type I) and IEC Standard 60296 (Type T).

The present invention further provides the use of the composition as hereinbefore described as an electrical insulting oil for application in one or more of transformers, regulators, circuit breakers, power plant reactors, shunt reactors, switch gears, cables and electrical equipment.

The present invention will be described with reference to the following Examples which are not intended to limit the scope of the invention in any way.

EXAMPLES

As the hydrorefined mineral oils, mineral oils and synthetic hydrocarbon oils, the base oils A-J and bright stock oil having the properties shown in Table 2, Table 3 and Table 4 were prepared.

Base Oils A-J and Bright Stock Oil

The properties of the base oils A-J and the bright stock oil are stated in Table 2, Table 3 and Table 4 on the basis of the following criteria.

-   -   (1) Flash point: as per JIS K 2265 (PMCC: Pensky-Martens closed         method, and COC: Cleveland open method).     -   (2) Kinematic viscosity (40° C.), kinematic viscosity (100° C.):         as per ASTM D445.     -   (3) Pour point: as per ASTM D97.     -   (4) Acid value: as per ASTM D974.

(5) PCA (PCA content): as per BS2000P-346. TABLE 2 Bright Base Base Base stock Units oil A oil B oil C oil Appearance Clear Clear Clear Clear Density g/cm³ 0.9040 0.8300 0.8324 0.9014 (15° C.) Flash point ° C. 146 140 200 324 (COC) Flash point ° C. 138 135 182 266 or (PMCC) more Kin. mm²/s 8.628 9.310 12.54 485.9 viscosity (40° C.) Kin. mm²/s 2.203 2.250 3.103 31.53 viscosity (100° C.) Pour point ° C. −55.0 −55.0 −24.0 −10.0 Acid value mg 0.00 0.00 0.01 0.01 KOH/g PCA content wt. % 1.1 0.0 0.2 1.4 Total wt. % ≦0.0005 ≦0.0001 0.0001 1.05 sulphur content Sulphide- wt. % ≦0.0005 ≦0.0001 ≦0.0001 0.39 type sulphur Mercaptan wt. % ≦0.0001 ≦0.0001 ≦0.0001 0.0041 sulphur Total ppm ≦1 ≦1 ≦1 235 nitrogen content Basic ppm ≦1 ≦1 ≦1 141 nitrogen content n-d-M ring Ca % 6.6 0.0 0.0 6.2 analysis Cn % 60.7 30.0 23.5 25.6 (ASTM D Cp % 32.7 70.0 76.5 68.2 3238) Base oil A B C D type Base oil types: A = hydrorefined naphthenic base oil (mineral oil) B = isoparaffinic base oil (synthetic oil) C = isoparaffinic base oil (synthetic oil) D = solvent-refined paraffin base oil (mineral oil)

TABLE 3 Base Base Base Base Base (Units) oil D oil E oil F oil G oil H Appearance clear clear clear clear clear Density g/cm³ 0.9067 0.8429 0.8401 0.8709 0.8470 (15° C.) Flash point ° C. 147 178 138 158 168 (COC) Kin. mm²/s 9.348 9.610 9.129 7.879 9.630 viscosity (40° C.) Kin. mm²/s 2.285 2.595 2.517 2.194 2.590 viscosity (100° C.) Pour point ° C. −55.0 −12.5 −12.5 −27.5 −12.5 Acid value mg 0.01 0.01 0.01 0.01 0.01 KOH/g Total wt. % 0.04 0.01 0.04 ≦0.01 0.28 sulphur content Total ppm 5 ≦1 ≦1 ≦1 ≦1 nitrogen content Basic ppm 4 ≦1 ≦1 ≦1 ≦1 nitrogen n-d-M ring Ca % 9.6 3.0 3.4 7.0 5.4 analysis Cn % 55.3 30.0 28.0 39.5 28.2 (ASTM D Cp % 35.1 67.0 68.6 53.5 66.4 3238) Base oil D E F G H type Base oil types: D = hydrorefined naphthenic base oil (mineral oil) E = isoparaffinic base oil (mineral oil) F = hydrorefined paraffinic base oil (mineral oil) G = hydrorefined paraffinic base oil (mineral oil) H = paraffinic base oil (mineral oil)

TABLE 4 (Units) Base oil I Base oil J Appearance clear clear Density (15° C.) g/cm³ 0.8184 0.8186 Flash point (COC) ° C. 234 226 Kin. viscosity (40° C.) mm²/s 16.10 16.40 Pour point ° C. −20.0 ≦50 Total sulphur content ppm ≦1 ≦1 Total nitrogen content ppm 1 ≦1 Basic nitrogen ppm ≦1 ≦1 n-d-M ring analysis Ca % 0.0 0.0 (ASTM D 3238) Cn % 8.8 10.0 Cp % 91.2 90.0 Base oil type I J Base oil types I = GTL (“XHVI”) base oil (synthetic oil) J = poly-α-olefin (synthetic oil)

Using the afore-mentioned base oils A-J and bright stock oil, Examples 1-7, which were electrical insulating oil compositions according to the present invention and not containing antioxidants, were prepared in accordance with the compositions shown in Table 5 and Table 6.

Furthermore, Examples 8-11, which are electrical insulating oil compositions according to the present invention and containing antioxidants, were prepared in accordance with the compositions shown in Table 7.

Further, for comparison with the afore-mentioned examples, Comparative Examples C1-C15 were prepared in accordance with the compositions shown in Tables 8-10.

With regard to the “clay treatment” referred to in Tables 5-10, activated clay in the stated quantity relative to the oil component was used and these were mixed together by stirring for 15 mins at 30-40° C., and the oil layer separated by pressure filtration. Processing was performed by the usual refining methods for electrical insulating oil compositions for the decolourisation, deodorisation, dewatering and stability improvement of electrical insulating oil compositions. TABLE 5 Example # Composition 1 2 3 4 5 Base oil A (wt. %) 80 80 80 60 80 Base oil B (wt. %) 16 — 17 — — Base oil C (wt. %) — 16 — — — Base oil D (wt. %) — — — 25 5 Base oil H (wt. %) — — — 13 13 Bright stock oil (wt. %) 4 4 3 2 2 Clay treatment * 1% 1% 1% 1% 1% * wt. % relative to amount of oil components used.

TABLE 6 Example # Composition 6 7 Base oil A (wt. %) 70.8 71.6 Base oil I (wt. %) 25.5 — Base oil J (wt. %) — 24.5 Bright stock oil (wt. %)  3.7  3.9 Clay treatment *  1%  1% * wt. % relative to amount of oil components used.

TABLE 7 Example # Composition 8 9 10 11 Base oil A (wt. %) 80 80 80 80 Base oil B (wt. %) 16 — 16 — Base oil C (wt. %) — 16 — 16 Bright stock oil (wt. %) 3.9 3.9 3.95 3.95 Antioxidant “BHT” (wt. %) 0.1 0.1 0.05 0.05 Clay treatment * 1% 1% 1% 1% Note: The antioxidant “BHT” is 2,6-di-tert-butyl-4-methylphenol. “BHT” added to oil components after clay treatment. * wt. % relative to amount of oil components used.

TABLE 8 Comparative Example # Composition C1 C2 C3 C4 C5 Base oil A (wt. %) 100 — — 80 80 Base oil B (wt. %) — — — 20 — Base oil C (wt. %) — — — — — Base oil D (wt. %) — — 100 — — Base oil E (wt. %) — — — — 20 Base oil F (wt. %) — — — — — Base oil G (wt. %) — 100 — — — Base oil H (wt. %) — — — — — Bright stock oil (wt. %) — — — — — Clay treatment * 1% 1% 1% 1% 1% * wt. % relative to amount of oil components used.

TABLE 9 Comparative Example # Composition C6 C7 C8 C9 C10 Base oil A (wt. %) 80 80 80 80 60 Base oil B (wt. %) — — — — — Base oil C (wt. %) — — — — — Base oil D (wt. %) — — — 20 40 Base oil E (wt. %) — — — — — Base oil F (wt. %) 20 — — — — Base oil G (wt. %) — 20 30 — — Base oil H (wt. %) — — — — — Bright stock oil — — — — — (wt. %) Clay treatment * 1% 1% 1% 1% 1% * wt. % relative to amount of oil components used.

TABLE 10 Comparative Example # Composition C11 C12 C13 C14 C15 Base oil A (wt. %) 70 70 60 80 90 Base oil B (wt. %) — — — — — Base oil C (wt. %) — — — — — Base oil D (wt. %) — — — — — Base oil E (wt. %) 15 20 27 — — Base oil F (wt. %) — — — — — Base oil G (wt. %) — — — — — Base oil H (wt. %) 15 10 13 20 10 Bright stock oil (wt. %) — — — — — Clay treatment* 1% 1% 1% 1% 1% *wt. % relative to amount of oil components used.

IEC standard values were used as the standard reference values for the performance of the electrical insulating oil compositions. If these standard reference values were satisfied, then it was considered that long-term stable use over a wide temperature range was possible.

Accordingly, for the afore-mentioned Examples 1 and 2 (not containing antioxidants), the values of the electrical insulating oil properties were measured, and those results are shown in Table 11 together with the standard reference values for IEC Standard 296, Class II. Further, for the afore-mentioned Examples 1, 2, 6 and 7 (not containing antioxidants), the values of the electrical insulating oil properties were measured, and those results are shown in Table 12 together with the standard reference values for IEC Standard 60296, Type U.

The measurement of the values of the properties, other than (1) to (5), described for the afore-mentioned base oils, was as stated below.

-   -   (6) Surface tension: as per ASTM D971.     -   (7) Corrosive sulphur: as per ASTM D1275.     -   (8) Water content: as per ASTM D1533.     -   (9) Oxidation test (168 hrs at 100° C.): as per IEC method         61125A.     -   (10) Oxidation test (164 hrs at 120° C.): as per IEC method         61125C.     -   (11) Insulation breakdown voltage: as per ASTM 1816 (VDE).

For the afore-mentioned Examples 3-5 (not containing antioxidants), the values of the density and oxidation stability on the basis of the aforesaid IEC Standard 296, Class II (168 hrs at 100° C., IEC method 61125A) were measured, and those results are shown in Table 13.

For the afore-mentioned Examples 8 and 9 (containing antioxidants), the values of the electrical insulating oil properties were measured, and those results are shown in Table 14 together with the standard values for IEC Standard 60296, Type I.

The measurement of the values of the properties, other than (1) to (11), described for the afore-mentioned base oils and Examples (not containing antioxidants), was as stated below.

-   -   (12) Oxidation test (500 hrs at 120° C.): as per IEC method         61125C.

Examples 10 and 11 (containing antioxidants) had lower antioxidant contents than Examples 8 and 9, and the values of their electrical insulating oil properties were measured, and those results are shown in Table 15 together with the standard values for IEC Standard 60296, Type T.

The measurement of the values of the properties, other than (1) to (11), described for the afore-mentioned oils and Examples (not containing antioxidants), was as stated below.

-   -   (13) Oxidation test (332 hrs at 120° C.): as per IEC method         61125C.

For Comparative Examples C1-C15, the afore-mentioned oxidation stability on the basis of the afore-mentioned IEC Standard 296, Class II (168 hrs at 100° C., IEC method 61125A) was measured, and those results are shown in Tables 16-18.

Discussion

For the non-antioxidant-containing electrical insulating oil compositions of Examples 1-7, the values relating to oxidation stability (IEC oxidation test), which is regarded as an important characteristic value, were substantially lower than the IEC Standard 296, Class II, standard reference values (maximum values) or the IEC Standard 60296, Type U standard reference values (maximum values), and said compositions were judged to be excellent as electrical insulating oil compositions.

With Examples 1 and 2, the values from the oxidation test by the IEC method 61125A (168 hrs at 100° C.) in the IEC Standard 296 and from the oxidation test by the IEC method 61125C (164 hrs at 120° C.) in the IEC Standard 60296 were substantially lower than the standard reference values (maximum values) in either of these standards, and the compositions were excellent.

Further, for Examples 1, 2, 6 and 7, favourable results were obtained for all of the characteristic values stated in Table 11 and Table 12, in particular the kinematic viscosity at 40° C. and −30° C., the pour point and acid value, and all compositions satisfied the standard reference values for IEC Standard 296, Type II and/or IEC Standard 60296, Type U.

Moreover, for the antioxidant-containing electrical insulating oil compositions of Examples 8 and 9, in the IEC method 61125C (500 hrs at 120° C.) oxidation test relating to IEC Standard 60296, Type I, the values relating to oxidation stability, which is regarded as an important characteristic value, were each substantially lower than the standard reference values (maximum values) therein, and they were judged to be excellent electrical insulating oil compositions. Further, favourable results were also obtained for the other characteristic values stated in Table 14, in particular the kinematic viscosity, with both compositions satisfying the standard reference values for IEC Standard 60296, Type I.

Furthermore, with the electrical insulating oil compositions of Examples 10 and 11, which contained small quantities of antioxidant, in the IEC method 61125C (332 hrs at 120° C.) oxidation test relating to IEC Standard 60296, Type T, the values relating to oxidation stability, which is regarded as an important characteristic value, were each substantially lower than the standard reference values therein, and said compositions were judged to be excellent electrical insulating oil compositions. Favourable results were also obtained for the other characteristic values stated in Table 15, in particular the kinematic viscosity, and both compositions satisfied the standard reference values for IEC Standard 60296, Type T.

However, with the substances of Comparative Examples C1-C15, in which bright stock oil was not used, in the IEC method 61125A (168 hrs at 100° C.) oxidation test relating to the afore-mentioned IEC Standard 296, none of the compositions satisfied the standard reference values for IEC Standard 296, Class II. In other words, the substances of Comparative Examples C1-C9 and C15 substantially exceeded the standard reference values (maximum values), and it was judged that the comparative compositions were unsuitable for use as electrical insulating oil compositions. Further, although the compositions of Comparative Examples 10-14 satisfied the standard reference values as regards acid value, said compositions did not satisfy the standard reference values as regards sludge, and it was judged that said comparative compositions could not be used as suitable electrical insulating oil compositions. TABLE 11 IEC Standard 296* Ex. 1 Ex. 2 Antioxidant not not not detected detected detected Kin. Viscosity 40° C. ≦11.0 10.06 10.73 (mm²/s) −30° C. ≦1800 1571 1389 Flash point ° C. ≧130 136 138 (PMCC) Pour point ° C. ≦−45 <−50 <−50 Appearance clear clear clear Density (20° C.) g/cm³ ≦0.895 0.8885 0.8890 Surface tension mN/m ≦40 46 46 (25° C.) Acid value mg KOH/g ≦0.03 0.00 0.00 Corrosive sulphur 140° C. × non- non- non- 19 hrs corrosive corrosive corrosive Water content mg/kg ≦30 10 10 (KF) Dielectric loss ≦0.005 0.0031 0.0028 tangent (90° C.) Oxidation test sludge ≦0.10 0.04 0.04 100° C. × (wt. %) 168 hrs (IEC acid value ≦0.40 0.17 0.15 method 61125A) (mg KOH/g) Insulation kV ≧30 80 80 breakdown voltage (VDE electrode) *Class II criteria Standard reference values

TABLE 12 IEC Standard 60296* Ex. 1 Ex. 2 Ex. 6 Ex. 7 Antioxidant not not not not not detected detected detected detected detected Kin. 40° C. ≦12 10.06 10.73 11.7 11.8 viscosity −30° C. ≦1800 1571 1389 1413 1307 (mm²/s) Flash point ° C. ≧135 136 138 145 143 (PMCC) Pour point ° C. ≦−40 <−50 <−50 −45 <−50 Appearance clear clear clear clear clear Density g/cm³ ≦0.895 0.8885 0.8890 0.8787 0.8796 (20° C.) Surface mN/m ≧40 46 46 46 46 tension (25° C.) Acid value mg ≦0.01 0.00 0.00 0.00 0.00 KOH/g Corrosive 140° C. × non- non- non- non- non- sulphur 19 hrs corrosive corrosive corrosive corrosive corrosive Water content mg/kg ≦30 10 10 10 10 (KF) Dielectric ≦0.005 0.0031 0.0028 0.0029 0.0027 loss tangent (90° C.) Oxidation Sludge ≦0.8 0.27 0.25 0.25 0.22 test (wt. %) 120° C. × acid ≦1.2 0.39 0.39 0.38 0.35 164 hrs value (IEC method (mg 61125C) KOH/g) Dielectric ≦0.500 0.180 0.184 0.170 0.175 loss Tangent (90° C.) PCA content wt. % ≦3 2.5 2.4 2.3 2.3 Insulation kV ≧30 80 80 80 80 breakdown voltage (VDE electrode) *Type U criteria Standard reference values

TABLE 13 IEC Standard 296* Ex. 3 Ex. 4 Ex. 5 Antioxidant not not not not detected detected detected detected Density g/cm³ ≦0.895 0.8877 0.8941 0.8942 (20° C.) Oxidation acid ≦0.40 0.15 0.18 0.17 test value 100° C. × (mg 168 hrs KOH/g) (IEC method sludge ≦0.10 0.08 0.06 0.04 61125A) (wt. %) *Class II criteria Standard reference values

TABLE 14 IEC Standard 60296 Ex. 8 Ex. 9 Antioxidant 0.08-0.4 0.1 0.1 wt. % wt. % wt. % Kin. viscosity 40° C. ≦12 10.04 10.68 (mm²/s) −30° C. ≦1800 1580 1385 Flash point ° C. ≧135 135 140 (PMCC) Pour point ° C. ≦−40° C. <−50 <−50 Appearance clear clear clear Density (20° C.) g/cm³ ≦0.895 0.8885 0.8890 Surface tension 25° C. ≧40 46 46 Acid value mg KOH/g ≦0.01 0.001 0.001 Corrosive sulphur 140° C. × non- non- non- 19 hrs corrosive corrosive corrosive Water content mg/kg ≦30 10 10 (KF) Dielectric loss ≦0.005 0.0016 0.0009 tangent (90° C.) Oxidation test sludge ≦0.8 0.52 0.47 120° C. × (wt. %) 500 hrs acid value ≦1.2 0.52 0.49 (IEC method (mg KOH/g) 61125C) Dielectric ≦0.500 0.245 0.117 loss tangent (90° C.) PCA content wt. % ≦3 2.5 2.4 Insulation kV ≧30 80 80 breakdown voltage (VDE electrode) * Type I criteria Standard reference values

TABLE 15 IEC Standard 60296* Ex. 10 Ex. 11 Antioxidant 0.01-0.08 0.05 0.05 wt. % wt. % wt. % Kin. viscosity 40° C. ≦12 10.13 10.77 (mm²/s) −30° C. ≦1800 1692 1405 Flash point ° C. ≧135 135 140 (PMCC) Pour point ° C. ≦−40 <−50 <−50 Appearance clear clear clear Density(20° C.) g/cm³ ≦0.895 0.8887 0.8892 Surface tension 25° C. ≧40 46 46 Acid value mg KOH/g ≦0.01 0.001 0.001 Corrosive sulphur 140° C. × non- non- non- 19 hrs corrosive corrosive corrosive Water content mg/kg ≦30 10 10 (KF) Dielectric loss ≦0.005 0.0016 0.0009 tangent 90° C. Oxidation test Sludge ≦0.8 0.43 0.44 120° C. × (wt. %) 332 hrs acid value ≦1.2 0.47 0.45 (IEC method (mg KOH/g) 61125C) Dielectric ≦0.500 0.215 0.160 loss tangent 90° C. PCA content wt. % ≦3 2.5 2.4 Insulation kV ≧30 80 80 breakdown voltage (VDE electrode) *Type T criteria Standard reference values

TABLE 16 IEC Standard Comp. Comp. Comp. Comp. Comp. Property 296* Ex. C1 Ex. C2 Ex. C3 Ex. C4 Ex. C5 Oxidation sludge ≦0.10 8.58 1.62 1.63 7.96 8.86 test (wt. %) 100° C. × acid value ≦0.40 17.21 1.41 0.92 12.09 11.40 168 hrs (mg KOH/g) (61125A method) *Class II criteria Standard reference values

TABLE 17 IEC Standard Comp. Comp. Comp. Comp. Comp. Property 296* Ex. C6 Ex. C7 Ex. C8 Ex. C9 Ex. C10 Oxidation sludge ≦0.10 5.95 6.84 5.67 1.35 0.28 test (wt. %) 100° C. × acid value ≦0.40 6.77 8.73 7.19 0.81 0.14 168 hrs (mg KOH/g) (61125A method) *Class II criteria Standard reference values

TABLE 18 IEC Standard Comp. Comp. Comp. Comp. Comp. Property 296* Ex. C11 Ex. C12 Ex. C13 Ex. C14 Ex. C15 Oxidation sludge ≦0.10 0.31 0.30 0.24 0.43 1.57 test (wt. %) 100° C. × acid value ≦0.40 0.12 0.13 0.15 0.14 1.01 168 hrs (mg KOH/g) (61125A method) *Class II criteria Standard reference values 

1. An electrical insulating oil having a flash point (PMCC) of 130° C. or more and a pour point of −40° C. or lower, comprising: an oil selected from the group consisting of hydrorefined mineral oils and synthetic hydrocarbon oils; and bright stock oil present in the range of from 0.5 to 10 wt. %, based on the total weight of the electrical insulating oil composition.
 2. The electrical insulating oil according to claim 1 wherein the bright stock oil has a total sulphur content in the range of from 0.4 to 2.0 wt. %, a total nitrogen content in the range of from 100 to 500 wt. ppm, and a basic nitrogen content of 300 wt. ppm or less.
 3. The electrical insulating oil composition according to claim 1, wherein said composition has a pour point of −45° C. or lower.
 4. The electrical insulating oil composition according to claim 1, wherein said composition has a flash point (PMCC) of 135° C. or more.
 5. The electrical insulating oil composition according to claim 1, wherein said composition comprises in the range of from 0.01 to 0.4 wt. % of phenolic antioxidant, based on the total weight of the electrical insulating oil composition.
 6. The electrical insulating oil composition according to claim 1, wherein said composition comprises in the range of from 0.01 to 0.08 wt. % of phenolic antioxidant, based on the total weight of the electrical insulating oil composition.
 7. The electrical insulating oil composition according to claim 1, wherein said composition comprises in the range of from 0.08 to 0.4 wt. % of phenolic antioxidant, based on the total weight of the electrical insulating oil composition.
 8. The electrical insulating oil composition according to claim 1, wherein the hydrorefined mineral oil has a kinematic viscosity in the range of from 8.0 to 24 mm²/s at 40° C. and is selected from the group consisting of hydrorefined naphthenic mineral oils and API base oil classification Group II or Group III hydrorefined paraffinic mineral oils.
 9. The electrical insulating oil composition according to claim 1, wherein the synthetic hydrocarbon oil has a kinematic viscosity in the range of from 8.0 to 24 mm²/s at 40° C. and is selected from the group consisting of isoparaffinic synthetic oils, GTL synthetic oils, poly-α-olefins (PAOs) belonging to API base oil classification Group IV, and a mixtures thereof.
 10. The electrical insulating oil composition according to claim 1, wherein the bright stock oil is a heavy mineral oil base oil fraction obtained by reduced pressure distillation of the residual paraffinic mineral oil from atmospheric distillation of crude oil, and deasphalting, solvent extraction and solvent dewaxing treatment of the residual oil obtained from said reduced pressure distillation.
 11. The electrical insulating oil composition according to claim 1, which composition conforms to at least one of the following Standards: IEC (International Electrotechnical Commission) Standard 296 (Class II), IEC Standard 60296 (Type U), IEC Standard 60296 (Type I) and IEC Standard 60296 (Type T).
 12. The electrical insulating oil according to claim 1 wherein the bright stock oil is a base oil belonging to API base oil classification Group I.
 13. The electrical insulating oil according to claim 11 wherein the bright stock oil has a total sulphur content in the range of from 0.4 to 2.0 wt. %, mercaptan sulphur content of 100 wt. ppm or less, and sulphide-type sulphur content of 0.5 wt. % or less.
 14. The electrical insulating oil according to claim 1 wherein the bright stock oil is a base oil has a total nitrogen content in the range of from 100 to 500 wt. ppm, and basic nitrogen content of 300 wt. ppm or less.
 15. The electrical insulating oil according to claim 1 wherein the bright stock oil is a base oil has a total content of polar components (IP368) in the range of from 30 to 70 wt. %. 